THIS invention relates to a shock absorber, and in particular to a shock absorber for use as a wagon retarder of the type in which movement of a wagon wheel along the rail is regarded by the retarder acting against a peripheral flange of the wheel.
In the applicant's corresponding U.S. Pat. No. 5,730,260, which is incorporated herein by reference in its entirety, a rail retarder-type shock absorber is disclosed which includes a cylinder defining a cavity between a pair of end walls. A piston rod extends axially through one of the end walls, and a piston is mounted on the piston rod between the end walls so as to divide the cavity into first and second working chambers containing a damping fluid. A first array of passages extends through the piston and communicates between the first and second chambers, and a sprung valve assembly is provided for obturating the first array of passages in the event of the velocity of the piston rod relative to the cylinder exceeding a predetermined value. A second array of passages communicates between the working chambers and provides a fluid flow path when the first array of passages is blocked by the sprung valve assembly. Restricted exhaust orifices communicate with the second array of passages for limiting the flow of fluid through the second array of passages so as to control the movement of the piston rod. The dimensions of the exhaust orifices are determined in accordance with the desired force-displacement profile of the shock absorber.
In a preferred embodiment, a so-called "relaxable orifice" version of the shock absorber is provided in the form of a moveable orifice plate which is spring biased against the rearmost face of the piston to define the restricted exhaust orifices and to allow them to open in response to an increase in pressure.
Whilst this shock absorber has been found to operate effectively, a number of manufacturing steps are associated with the formation of the first and second arrays of passages, as well as the formation and adjustment of the restricted exhaust orifices.
In addition, in the "relaxable orifice" version, it has been proved difficult to release sufficient fluid as the orifice plate opens to ensure that the counter-force exerted by the retarder is insufficient to cause bounce and possible derailment in the case of lighter rolling stock moving at relatively high speeds.